This invention relates to an electronic interface circuit, and more particularly to a fully electronic interface circuit for coupling an electronic system to a telephone communications system.
With the advent of remote data processing and telecommunications sytems and equipment, it has become desirable to couple electronic equipment and devices to a telephone communications system. In present day telephone communications systems, connection to the telephone system requires an interface circuit or device that couples an electronic system to a pair of conductors from the telephone system. In the past, the device used to connect an electronic system to the conductors of a telephone communications system has often been a coupling transformer (or other inductive device) used to match the electrical characteristics of the telephone system. However, inductive devices have certain disadvantages. For example, they may not provide the correct impedance matching over the entire voice band. In addition, signal coupling transformers used in communications systems are generally larger and more expensive than desirable, particularly where the electronic system to be coupled to the telephone line is designed to be handheld.
The prior art has sought to develop electronic coupling circuitry to replace such inductive devices. For example, U.S. Pat. No. 4,214,130 to Questad shows an electronic gyrator circuit for use in coupling an electronic system to a pair of communications lines. However, the invention shown in Questad permits signal coupling in one direction only, from the electronic system to the telephone system. Other prior art gyrators have contained numerous parts or have circuits using relatively expensive parts, either because great precision is needed in the values of the components, or because high power handling capabilities are necessary.
In coupling an electronic system to a telephone system, care must be taken to protect the electronic system from transient voltage and current surges. Such surges may be induced by lightning strikes on the telephone system lines. Further, the voltage and current characteristics of telephone systems vary widely. In the United States, for example, the loop current of a telephone system may range from approximately 20 milliamps to 90 milliamps. The voltage may range from approximately 42 volts to 60 volts, and the AC line impedance may range from 400 to 1300 ohms. Indeed, it is not unknown for any of these values to vary even beyond these ranges. Further, the DC polarity of the telephone line may be reversed from its nominal polarity. It is important to devise an interface system that will be compatible with and protect against these variations in the telephone system circuit characteristics, since the electronic system may be vary sensitive to changes in these parameters.
It is also often desirable to use the power available from a telephone system to power an electronic system. Because of the relatively small current available from a telephone system, an electronic system designed to be telephone-line powered must be carefully designed for low power consumption. With modern electronics, this goal is not difficult if the electronic system is not required to directly drive the telephone line. The telephone system typically has an impedance of 600 ohms. A typical low power electronic system may have a relatively high impedance, such as 20,000 ohms. Thus an impedance converter circuit is needed to match the impedance of the electronic system with the impedance of the telephone system. In the past, such impedance conversion typically has been performed through the use of relatively bulky and expensive transformers.
In many cases, it is also desirable to couple an electronic system to a telephone system using a fully controllable interface. Thus, it would be convenient and desirable to replace the mechanical hookswitch of a typical telephone system coupling circuit with a fully electronic hookswitch controllable by a remote manual switch or by electronic programmed control.
Therefore, it is an object of this invention to provide a new and improved electronic telephone interface circuit. More precisely, the functions that such an invention should achieve are as follows:
To provide a fully electronic, inductorless, bi-directional circuit suitable for coupling communications and voice signals from an electronic system to a pair of conductors of a telephone communications system.
To provide for an impedance conversion between a telephone system and an electronic system.
To eliminate bulky and expensive inductors.
To provide a small and inexpensive coupling means for bi-directionally coupling an electronic system to the conductors of a communications system.
To protect an electronic system from the telephone system in order to minimize or eliminate any damage to the electronic system from voltage or current surges.
To protect the electronic system from variations in the nominal electrical characteristics of the telephone system.
To replace the mechanical hookswitch in the coupling circuit between an electronic system and a telephone system with a remotely controllable electronic switch.
To provide an interface in compliance with FCC Part 68 (concerning standards for interfacing to the public telephone system) that will also withstand without damage line surges as outlined in Part 68.
The present invention achieves all of the above functions with a simple and inexpensive electronic circuit, described in full below.